MFWDs are usually individually adapted to specific functions or needs of a certain type of users. In some cases, it may be desirable that the MFWD is either e.g. small while in other cases this is not of importance since e.g. a keyboard or screen is provided by the MFWD which already requires a certain size.
Many of the demands for modern MFWDs also translate to specific demands for the antennas thereof. For example, one design demand for antennas of multifunctional wireless devices is usually that the antenna be small in order to occupy as little space as possible within the MFWD which then allows for smaller MFWDs or for more specific equipment to provide certain function of the MFWD. At the same time, it is sometimes required for the antenna to be flat since this allows for slim MFWDs or in particular, for MFWDs which have two parts that can be shifted or twisted against each other.
In the context of the present application, a device is considered to be slim if it has a thickness of less than about 14 mm, 13 mm, 12 mm, 11 mm, 10 mm, 9 mm or 8 mm. A slim MFWD should be mechanically stable, mechanical stability being more difficult to achieve in slim devices.
Additionally, antennas in some embodiments are required to be multi-band antennas and to cover different frequency bands and/or different communication system bands. Beyond that, some of the bands have to be particularly broad like the UMTS band which has a bandwidth of 12.2%. For a good wireless connection, high gain and efficiency are further required. Other more common design demands for antennas are the voltage standing wave ratio (VSWR) and the impedance which is typically about 50 ohms.
Furthermore of particular importance, is omni-directional coverage which means that the antenna radiates with a substantially donut-shaped radiation pattern such that e.g. terrestrial base stations of mobile telephone communication systems can be contacted within any direction in the horizontal plane.
However, for satellite communication (for example, for receiving GPS-signals), other radiation patterns are preferred, in particular, those which radiate into the upper hemisphere. Here radiation into the horizontal plane is usually less desired. The polarization of the emitted or received radiation also has to be taken into consideration. Other demands for antennas for modern MFWDs are low cost and a low specific absorption rate (SAR).
Furthermore, an antenna has to be integrated into a device such as MFWD such that an appropriate antenna may be integrated therein which puts constraints upon the mechanical fit, the electrical fit and the assembly fit of the antenna within the device. Of further importance, usually, is the robustness of the antenna which means that the antenna does not change antenna properties in response to smaller shocks to the device.
As can be imagined, a simultaneous improvement of all features described above is a major challenge for persons skilled in the art. A typical exemplary design problem is the generally uniform line of thinking that due to the limits of diffraction, a substantial increase in gain and directivity can only be achieved through an increase in the antenna size.
On the other hand, a MFWD that has a high directivity and hence, a high gain, has to be properly oriented towards a transceiver-base station. This, however, is not always practical since portable device users need to have the freedom to move and change direction with respect to a base station without losing coverage and, therefore, losing the wireless connection. Therefore, less gain is usually accepted in order to obtain an omni-directional (donut-like) radiation pattern.
It has to be taken into account that a palmtop, laptop, or desktop portable device might require a radiation pattern that enhances radiation in the upper hemisphere, i.e., pointing to the ceiling and the walls rather than pointing to the floor, since transceiver stations such as a hotspot antenna or a base station are typically located above or on the side of the portable device. If, however, such a device is used for a voice phone call it will be held substantially upright close to the user's head in which case an omni-directional pattern is preferred which is oriented so that the donut-like shape of the radiation pattern lies in the horizontal.
While it might appear desirable to provide an antenna with a uniform radiation pattern (sphere-like) for voice calls such a pattern turns out to have substantial drawbacks in terms of a desired low specific absorption rate since it sometimes leads to an increased absorption of radiation within the hand and the head of the user during a voice phone call.
In every MFWD, the choice of the antenna, its placement in the device and its interaction with the surrounding elements of the device will have an impact on the overall wireless connection performance making its selection non-trivial and subject to constraints due to particular target use, user and market segments for every device.
As established by L. J. Chu in “Physical Limitations of Omni-Directional Antennas”, Journal of Applied Physics, Vol. 19, December, 1948, pg. 1163-1175, and Harold A. Wheeler, in “Fundamental Limitations of Small Antennas”, Proceedings of the I.R.E., 1947, pgs. 1479-1488. small antennas may not exceed a certain bandwidth. The bandwidth of the antenna decreases in proportion to the volume of the antenna. The bandwidth, however, is proportional to the maximum data rate the wireless connection can achieve and, therefore, a reduction in the antenna size is additionally linked to a reduction in the speed of data transmission.
Furthermore, a reduction of the antenna size can be achieved, for example, by loading the antenna with high dielectric materials for instance by stuffing, backing, coating, filling, printing or over-molding a conductive antenna element with a high dielectric material. Such materials tend to concentrate a high dielectric and magnetic field intensity into a smaller volume. This concentration leads to a high quality factor which, however, leads to a smaller bandwidth. Further, such a high concentration of electromagnetic field in the material leads to inherent electrical losses. Those losses may be compensated by a higher energy input into the antenna which then leads to a portable wireless device with a reduced standby or talk/connectivity time. In the design of MFWDs, every micro Joule of energy available in the battery has to be used in the most efficient way.
Multi-band antennas require a certain space since for each band a resonating physical structure is usually required. Such additional resonating physical structures occupy additional space which then increases the size of the antenna. It is therefore particularly difficult to build antennas which are both small and multi-band at the same time.
As already mentioned above, there exists a fundamental limit established by Chu and Wheeler between the bandwidth and antenna size. Therefore, many small antennas have great difficulty in achieving a desired large bandwidth.
Broadband operation may be achieved by two closely neighboring bands which then require additional space for the resonating physical structure of each of the bands. Further, those two antenna portions may not be provided too close together since, due to electric coupling between the two elements, the merging of the two bands into a single band is not achieved, but rather splitting the resonant spectrum into independent sub-bands which is not acceptable for meeting the requirements of wireless communication standards.
Furthermore, for broadband operation the resonating physical structure needs a certain width. This width, however, requires additional space which further shows that small broadband antennas are difficult to achieve.
It is known to achieve a broadband operation with parasitic elements which, however, require additional space. Such parasitic elements may also not be placed too close to other antenna portions since this will also lead to splitting the resonant spectrum into multiple sub-bands.
An antenna type which may be particularly suitable for slim multifunctional devices or those composed of two parts which can be moved against each other (such as twist, clamshell or slide devices) is a patch antenna (and particularly a PIFA antenna). However patch antennas, are unfortunately known to have poor gain and narrow bandwidths, typically in the range of 1% to 5% which is unsuitable for coverage of certain bands such as the UMTS band.
Although it is known that the bandwidth may be increased by changing the separation between the patch and its ground plane, this then destroys the advantage of patch antennas being flat. This also leads to a distortion of the radiating pattern, for instance, due to surface wave effects.
For patch antennas it is known that by providing a high dielectric material between the patch and the ground plane, it is possible to reduce the antenna size. As mentioned above, such high dielectric materials tend to reduce the bandwidth which is then disadvantageous for patch antennas. Such materials also generally increase losses.
Further difficulties in antenna design occur when trying to build multi-band antennas. While it is possible to separate different antenna portions from each other with appropriate slots or the like, currents and charges in the respective parts always interact with one another by strong and far-reaching electromagnetic fields. Those different antenna branches are, therefore, never completely independent of one another. Trying to add a new branch to an existing antenna structure to produce a new antenna frequency of resonance therefore changes entirely the previous antenna frequencies. Therefore, it is difficult to simply take a working antenna and try to add one more band by just adding one more antenna portion. All previously achieved optimizations for already established frequency bands are lost by such an approach.
Trying to design an antenna with three or more bands gives rise to a linear or, in the worst case an exponential, rise in the number of parameters to consider or problems to resolve. For each band, resonant frequency, bandwidth, and other above-mentioned parameters such as impedance, polarization, gain, and directivity must all be controlled simultaneously. Furthermore, multi-band antennas may be coupled with two or more radio frequency devices. Such coupling raises the issue of isolation between the different radio frequency devices, which are both connected to the same antenna. Isolation of this type is a very difficult task.
Physical changes intended to optimize one parameter of one antenna band change other antenna parameters, most likely in a counter-productive way. It is usually not obvious how to control the counter-productive effects or how to compensate for them without creating still more problems.
Mechanical considerations must also be taken into account in antenna design. For example, the antenna needs to be firmly held in place within a device. However, the materials that are in very close proximity to the metal piece or the conductive portion which forms an antenna or antenna portion, have a great impact on the antenna characteristics. Sometimes extensions or small recesses in the metal piece are provided to firmly hold the antenna in place, however such means which are intended for giving mechanical robustness to the antenna also interact with and change the electric properties of the antenna.
All these different design problems of antennas may only be solved in the design of the geometry of the antenna. All parameters such as size, flatness, multi-band operation, broadband operation, gain, efficiency, impedance, radiation patterns, specific absorption rate, robustness and polarization are highly dependent on the geometry of the antenna. Nevertheless, it is practically impossible to identify at least one or two geometric features which affect only one or two of the above-mentioned antenna characteristics. Thus, there is no individual geometry feature which can be identified in order to optimize one or two antenna characteristics, without also influencing all other antenna characteristics.
Any change to the antenna geometry may harm more than it helps without knowing in advance how and why it happens or how it can be avoided.
Additionally, every platform of a wireless device is different in terms of form factor, market and technical requirements and functionality which requires different antennas for each device.
One problem is solved by providing the MFWD with an RF system and an antenna system with the capability of fully functioning in one, two, three or more communication standards (such as e.g. GSM 850, GSM 900, GSM 1800, GSM 1900, UMTS, CDMA, W-CDMA, etc.), and in particular mobile or cellular communication standards, each standard allocated in one or more frequency bands, each of said frequency bands being fully contained within one of the following regions of the electromagnetic spectrum:                the 810 MHz-960 MHz region,        the 1710 MHz-1990 MHz region,        and the 1900 MHz-2170 MHz regionsuch that the MFWD is able to operate in three, four, five, six or more of said bands contained in at least said three regions.        
One problem to be solved by the present invention is therefore to provide an enhanced wireless connectivity. Another effect of the invention is to provide antenna design parameters that tend to optimize the efficiency of an antenna for a MFWD device while observing the constraints of small device size and enhanced performance characteristics.